Data networks

ABSTRACT

A LAN or other data network includes at least first and second repeater units for receiving data signals and repeating them to other parts of the data network and a transceiver unit for transmitting and receiving data which is connected to the first repeater unit by a primary data link and to the second repeater unit by a secondary data link. The primary data link comprises a primary transmit line for transmitting data from the transceiver unit to the first repeater unit and a primary receive line for receiving data from the first repeater unit and transmitting it to the transceiver unit; and the secondary data link comprising a secondary transmit line for transmitting data to the second repeater unit and a secondary receive line for receiving data from the second repeater unit and transmitting it to the transceiver unit. Intelligent monitoring means determines whether or not the transceiver unit is transmitting data and also whether or not data signals are present on each of the primary receive line and the secondary receive line and accordingly controls a switch unit to switch between two states in which either the primary transmit and receive lines or the secondary transmit and receive lines, but not both, are in communication with the transceiver unit. Both transmit and receive faults can be detected and appropriate action to avoid them taken.

This is a continuation of Ser. No. 07/836,748, filed Feb. 19, 1992, nowabandoned, which is a continuation of Ser. No. 07/525,721, filed May 21,1990, now U.S. Pat. No. 5,115,449.

The invention relates to data networks and relates primarily, but notexclusively, to local area data networks.

The successful transmission of data in a local area network is dependenton the existence of a data transfer path between the sending andreceiving stations. In a large local area network the path between thesending and receiving station may consist of a large number of linksconstituted by a variety of active or passive network components. It isapparent that any one of these interlinking components may fail, andconsequently disrupt data transmission between the sending and receivingstation.

The present invention provides a data network in which failures can beidentified and in which an alternative route for data traffic will beautomatically provided.

Accordingly there is provided a data network including

at least first and second repeater units for receiving data signals andrepeating them to other parts of the data network;

a transceiver unit for transmitting and receiving data;

a primary data link comprising a primary transmit line for transmittingdata from the transceiver unit to the first repeater unit and a primaryreceive line for receiving data from the first repeater unit andtransmitting it to the transceiver unit;

a secondary data link comprising a secondary transmit line fortransmitting data from the transceiver unit to the second repeater unitand a secondary receive line for receiving data from the second repeaterunit and transmitting it to the transceiver unit; and intelligentmonitoring means for determining whether or not the transceiver unit istransmitting data and also whether or not data signals are present oneach of the primary receive line and the secondary receive line; and

a switch unit actuated by signals from the said monitoring means toswitch between two states in which either the primary transmit andreceive lines or the secondary transmit and receive lines, but not both,are in communication with the transceiver unit.

It should be noted that the transceiver referred to may itself be arepeater unit.

The invention therefore provides a primary data link between thetransceiver unit and the first repeater unit, and a secondary data linkbetween the transceiver unit and the second repeater unit, themonitoring means being capable of switching between the two alternativedata links but always keeping one link open.

When the transceiver unit is not transmitting but receiving data innormal operation of the data network, the data should normally bereceived from both the first and second repeater units and thus appearon both the primary and secondary receive lines. If data is present onone line but not on the other, it is reasonable to assume that there isa fault affecting the data link of which the receive line having no dataon it forms part, or the repeater to which that link is connected.Preferably therefore, the monitoring means is programmed to detect afirst condition in which the transceiver unit is not transmitting dataand there is data present on only one of the primary and secondaryreceive lines and to respond to that first condition by actuatingexclusively the data link including the receive line on which data waspresent when the said first condition was detected.

The monitoring means should only actuate the switch unit if the firstcondition has been continuously detected for a predetermined timeperiod. In this way should a data signal appear on one receive linebefore it appears on the other, due to normal operating delays withinthe data network, a fault condition is not diagnosed in error.

When the transceiver unit is transmitting to one repeater unit, thetransmitted signal should eventually reappear on the receive line fromthe other repeater unit. Failure to re-appear may be due to a fault inthe data link associated with the transmitting of data to the onerepeater unit, or the data link associated with the receiving of datafrom the other repeater unit or to other faults, e.g. in a data linkbetween the two repeaters. Once one of the receive lines is seen to beoperational, by the presence of data on it when the transceiver unit isno longer itself transmitting, it is reasonable to work on the basisthat the fault is associated with the transmit line. Preferablytherefore the monitoring means is also programmed to detect a secondcondition, namely when a first observation shows that the transceiverunit is transmitting data on the transmit line of either the primary orthe secondary data link and there is no data on the receive line of theother data link, and then a second observation, within a predeterminedtime period from the first observation, shows that there is data on thesaid other receive line when the transceiver is not transmitting dataand to respond to that second condition by actuating the switch unit, ifnecessary, to bring the transceiver unit into communication with thedata link including the receive line on which data was observed in thesecond said observation but not in the first.

The monitoring means should only respond to the second condition if inthe first observation the absence of data from the relevant receive linkhas been continuously detected for a predetermined time period. Thisavoids the spurious diagnosis of a fault before the transmitted datasignal has had time to pass around the data network and re-appear at thereceive line from the other repeater unit.

It will be seen that receive line faults are detected because they giverise to the first condition, and transmit line faults because they giverise to the second condition.

Whether the fault is diagnosed as being associated with the transmit orreceive lines, the monitoring means preferably also sends a signal toactuate an error indicator in a network manager or elsewhere.

Additionally or alternatively, there is conveniently provided means forresetting the switch unit to its original state after a predeterminedperiod of time has elapsed following the detection of the first orsecond condition. This causes the intelligent monitoring means tore-check the line that was diagnosed faulty and prevents a temporaryfault which is self correcting or a fault in the network beyond thefirst and second repeaters from causing the processing unit to selectexclusively for an indefinite period either the primary or secondarydata links.

Preferably the monitoring means is also programmed to detect a thirdcondition, namely when the transceiver unit is transmitting data on thetransmit line of its primary or secondary data links and there is dataon the receive line of its other data link and to actuate the switchunit when the said third condition ceases to change the data link towhich the transceiver is connected. In this way, when no fault has beendiagnosed, the monitoring means switches between the primary andsecondary data links after each transmission by the transceiver unit.Alternatively or additionally, the monitoring means switches between theprimary and secondary lines after a predetermined interval, providedthat the transceiver unit is neither transmitting nor receiving. Ineither case, the transceiver unit thereby transmits alternately over theprimary and secondary lines, which are therefore regularly monitored,and there are advantages over systems in which a back-up line is onlybought into operation in the event of a fault and in which problems inthe back-up line may remain undetected until the very time that theback-up line is needed.

To gain maximum benefit from the invention, the first and secondrepeater units should be wholly independent of one another, but someadvantage can be obtained even if they are separate channels in a singlemultiport repeater.

It is considered an advantage of the invention that the monitoring meansand switching unit may be located together adjacent to or inside thetransceiver that they serve; but it is possible in principle to locatethe switching unit and optionally part of the monitoring means at theopposite end of the data links, or at some intermediate point, providedreliable communication of monitoring signals can be ensured.

The present invention also extends to a method of operating a datanetwork including a transceiver unit for transmitting and receivingdata, and at least first and second repeater units for receiving datasignals from that transceiver unit and repeating them to other parts ofthe data network, the method comprising the steps of providing a primarydata link comprising a primary transmit line for transmitting data fromthe transceiver unit to the first repeater unit and a primary receiveline for receiving data from the first repeater unit and transmitting itto the transceiver unit; providing a secondary data link comprising asecondary transmit line for transmitting data from the transceiver unitto the second repeater unit and a secondary receive line for receivingdata from the second repeater unit and transmitting it to thetransceiver unit; determining whether or not the transceiver unit istransmitting data; determining the presence or absence of data signalson the primary receive line and the secondary receive line; andswitching between two states in which respectively the primary data linkand the secondary data link is in communication with the transceiverunit, depending on whether the transceiver unit is transmitting data andwhether there are data signals on the primary receive line and thesecondary receive line.

The invention will now be further described, by way of example only,with reference to the accompanying drawings, in which;

FIG. 1 is a schematic diagram of a conventional data network;

FIG. 2 is a schematic diagram of a part of a data network according tothe present invention;

FIG. 3 is a flow diagram illustrating the decision logic of the part ofthe data network of FIG. 2;

FIG. 4 is a schematic diagram of a data network incorporating theapparatus illustrated in FIG. 2; and

FIG. 5 is a schematic diagram of an alternative layout of data networkincorporating the apparatus of FIG. 2.

Referring to FIG. 1, a data network employing a CSMA/CD (carrier sensemultiple access with collision detection) access method comprises anumber of pieces of data terminal equipment, such as computers etc,illustrated in FIG. 1 as DTE's 1 to 15. Each DTE has access to the datanetwork via a media access unit, illustrated as MAU's 1 to 15respectively. Each group of MAU's is associated with a multiportrepeater (MPR), MAU's 1 to 5 with MPR 1, MAU's 6 to 10 with MPR 2 andMAU's 11 to 15 with MPR 3. MPR's 1 to 3 are themselves interlinked viaMPR 4, and it will be seen that the network could be further extended inany conventional or convenient way.

The operation of the network of FIG. 1 will now be briefly described.Data from DTE 1 is passed to MPR 1 via MAU 1 and via bidirectional datalink 21, and the multiport repeater is designed to repeat the datasignals to each of MAU's 2 to 5 via bidirectional data links 22-25.Additionally MPR 1 sends the data via a further bidirectional data link26 to MPR 4, which itself repeats the data to MPR's 2 and 3, and onwardsto each of MAU's 6 to 15 by corresponding data links. In this wayinformation transmitted by DTE 1 is repeated to every other DTE in thenetwork. The network will operate similarly with data from any of theDTE's.

It will be seen from FIG. 1 that data from any of DTE's 1 to 5 destinedfor any of DTE's 6 to 15 must pass through MPR 4. Consequently a failurein MPR 4, or in the data links connecting it with other MPR's will havea particularly disruptive effect on the data network.

FIG. 2 shows first and second MPR's 31 and 32 both connected to a DTE 33by intelligent processing unit 34 in the form of a modified MAU,hereinafter referred to as a dual MAU or DMAU. The DMAU 34 is connectedto MPR 31 by a bidirectional data link (the primary data link) made upof primary transmit line PT and primary receive line PR; it is alsoconnected to MPR 32 by another bidirectional data link (the secondarydata link) made up of transmit line ST and secondary receive line SR.These transmit and receive lines are preferably optical fiber links ortwisted pair electrical cable, but other links (including optical linksin free space or coaxial cable) could be used if desired. Appropriateconnection points 35 for PT, PR, ST and SR are provided within the DMAU.

Also within the DMAU 34 is an electronic switch unit SU which alwaysconnects either the two primary lines PT and PR or the two secondarylines ST and SR to the corresponding one of receive line R and transmitline T which are connected via interface unit 36 and bidirectional datalink 37 to the DTE 33. Alternatively DMAU 34 could be incorporatedwithin the DTE. The DMAU 34 also comprises a monitoring unit 38 whichreceives data signals from receive lines PR and SR on lines 9 and 10respectively and from transmit line T on line 11.

The monitoring unit 38 determines the presence or absence of datasignals on lines PR, SR and T by means of their appearance or otherwiseon lines 9, 10 and 11. Depending on the combination of signals detectedby the monitoring unit 38, signals may be sent via line 12 to the switchunit SU to switch between a primary connection in which PT and PR areconnected to T and R respectively, and a secondary connection in whichST and SR are connected to T and R respectively. The logic operated bythe monitoring unit is illustrated in FIG. 3 and set out in thefollowing table, in which the three "condition" columns represent thepresence or absence of data on lines T, PR and SR respectively accordingto the convention

1=at some time during the expected time period of a transmission, datahas been present

0=throughout the expected time period of a transmission data has beenabsent

X=logic is independent of the presence of absence of a signal on thisline in this state.

By the "expected time period" is meant the length of time from thebegining of a transmission to the end of its reception back at its pointof origin by the slowest route: this period is charateristic of thenetwork, and can be taken as 2 milliseconds for a single networkoperating Ethernet protocol.

                  TABLE                                                           ______________________________________                                        Condition                                                                     T   PR     SR      Representing  Transition                                   ______________________________________                                        State 1 - normal operation on primary data link                               1   0      1       normal transmission                                                                         to state 2                                   0   0      1       primary receive fault                                                                       to state 8,                                                                   switch to secondary                          0   1      0       secondary receive fault                                                                     to state 3                                   1   X      0       fault on PT or SR                                                                           to state 4                                   Any other  normal operation                                                                              none                                               State 2 - normal switchover to secondary                                      0   0      0       transmission ended                                                                          to state 6, switch                                                            to secondary                                 Any other  still transmitting                                                                            none                                               State 3 - secondary receive fault, operation on primary                       data link                                                                     0   0      0 on or             to state 1 to                                  after timeout          retest                                                 State 4 - primary transmit or secondary receive fault                         X   X      1       secondary receive OK                                                                        to state 10, switch                                                           to secondary                                 0   0      0 on or               to state 1, to                               after timeout          retest                                                 State 5 - secondary transmit fault, operation on primary                      data link                                                                     0   0      0 on or             to state 1                                     after timeout          to retest                                              State 6 - normal operation on secondary data link                             1   1      0       normal transmission                                                                         to state 7                                   0   1      0       secondary receive fault                                                                     to state 3,                                                                   switch to primary                            0   0      1       primary receive fault                                                                       to state 8                                   1   0      X       fault on ST or PR                                                                           to state 9                                   Any other  normal operation                                                                              none                                               State 7 - normal switchover to primary                                        0   0      0       transmission ende                                                                           to state 1,                                                                   switch to primary                            Any other  still transmitting                                                                            none                                               State 8 - primary receive fault, operation on secondary data                  link                                                                          0   0      0 on or             to state 6 to                                  after timeout          retest                                                 State 9 - secondary transmit or primary receive fault                         X   1      X       primary receive OK                                                                          to state 5, switch                                                            to primary                                   0   0      0 on or               to state 6                                   after timeout          to retest                                              State 10 - primary transmit fault, operation on secondary                     data link                                                                     0   0      0 on or             to state 6 to                                  after timeout          retest                                                 ______________________________________                                    

In the table, "timeout" indicates the passing of an arbitrary timeperiod since entry into the state concerned; a timeout period of 60seconds is considered suitable.

It will be appreciated that as an alternative to monitoring data on lineT, the lines PT and ST could be input to the monitoring means in orderto establish whether DTE 3 is transmitting data onto the network.

FIGS. 4 and 5 show two alternative ways in which the DMAU 34 of FIG. 2can be used in a data network. In FIG. 4 a DMAU is placed between eachDTE and two multiport repeaters. In this way a failure of an MPR, or anyof the bidirectional links between the DMAU's and the MPR's, will causea switchover to an alternative MPR and maintain each of the DTE's incommunication one with another. FIG. 5 shows a less sophisticatedarrangement in which the DMAU's are placed between one level of MPR's inthe network and another. This will provide alternative data routes inthe event of a failure of any of the MPR's or the links between them,and if a fault does occur in an area of the network unprotected byDMAU's, only a single DTE or at worst the DTE's connected to a singleMPR will be out of communication.

What we claim our invention is:
 1. A transceiver for coupling a terminalto a local area network across first and second data links, comprising:afirst pod for connection to the first data link and a second port forconnection to the second data link to couple both the first and secondports to the local area network; a switch unit, having a first stateproviding for communication between the terminal and the first port forreceiving and transmitting data signals on the local area network, and asecond state providing for communication between the terminal and thesecond port for receiving and transmitting data signals on the localarea network; and a monitoring circuit, coupled to the switch unit andthe first and second ports which detects a current state of the switchunit, and conditions of the transceiver, including transmission of datasignals through the transceiver, and the presence or not of data signalsfrom the network on the first and second ports, and controls the switchunit in response to the current state of the switch unit and thedetected conditions of the transceiver.
 2. A transceiver as claimed inclaim 1 in which said monitoring circuit detects a first condition inwhich there is no data transmission from the terminal and data from thenetwork is present on only one of the said first and second ports, andresponds to that first condition by controlling the switching unit touse a state providing for communication across the one port on whichdata from the network was present when the said first condition wasdetected.
 3. A transceiver as claimed in claim 1 in which the monitoringcircuit detects a second condition, namely when a first observationshows that there is transmission of data from the terminal on eithersaid first or said second port and data from the network is not presenton the other port, and then a second observation, within a predeterminedtime period from said first observation, shows that data from thenetwork is present on said other port when there is no transmission ofdata from the terminal, and responds to that second condition bycontrolling said switch unit to use a state providing for communicationthrough said other port on which data from the network was present inthe second said observation but not in the first said observation.
 4. Atransceiver as claimed in claim 2 in which the network includes an errorindicator and said monitoring circuit on detecting said first conditionalso sends a signal to actuate the error indicator.
 5. A transceiver asclaimed in claim 3 in which the network includes an error indicator andsaid monitoring circuit on detecting the said second condition sends asignal to actuate the error indicator.
 6. A transceiver as claimed inclaim 2 including means for controlling said switch unit after apredetermined period of time has elapsed following the detection of saidfirst condition to change states.
 7. A transceiver as claimed in claim 3including means for controlling said switch unit after a predeterminedperiod of time has elapsed following the detection of said secondcondition to change states.
 8. A transceiver as claimed in claim 1 inwhich said monitoring circuit detects a third condition, namely whenthere is transmission of data from the terminal on one of the first orsecond ports and there is data from the network present on the otherport, and to actuate the switch unit to change states when said thirdcondition ceases.
 9. A transceiver as claimed in claim 2 in which saidmonitoring circuit controls said switch unit to change states so thatcommunication switches between said first and second ports after apredetermined interval from the detection of the said first condition,provided that the terminal is not transmitting, nor is data from thenetwork present on any of the first and second ports.
 10. A transceiveras claimed in claim 3 in which said monitoring circuit controls saidswitch unit to change states so that communication switches between saidfirst and second ports after a predetermined interval from the detectionof said second condition, provided that the terminal is nottransmitting, nor is data from the network present on the first andsecond ports.